It has been proposed to mount radiators on the roof of a vehicle, a representative approach being disclosed in U.S. Pat. No. 2,784,568 to Schjolin in which the entire air conditioning system as well as the radiator are mounted beneath the roof line and directly above the passenger compartment in order to conserve space. Another approach is disclosed in U.S. Pat. No. 6,910,529 to Stone et al wherein the radiator assembly is mounted on the roof and has fans or blower units mounted above the radiator sections. Among other advantages, separation of the radiator assembly from the engine and mounting along the roof line enables the heat to be dissipated from the radiator and exposes the radiator to cleaner air while at the same time making the engine more accessible and easier to cool. Nevertheless, there is a previously unmet need for a radiator assembly which can be mounted above the roof line while occupying a minimum of space and maintaining a low profile which will permit it to be mounted with the existing roof-mounted air conditioning systems, such as, those of the type disclosed in my U.S. Pat. No. 4,607,497 for ROOF-MOUNTED AIR CONDITIONER SYSTEM HAVING MODULAR EVAPORATOR AND CONDENSER UNITS, U.S. Pat. No. 4,679,616 for ROOF-MOUNTED AIR CONDITIONER SYSTEM HAVING MODULAR EVAPORATOR AND CONDENSER UNITS, U.S. Pat. No. 5,285,654 for EVAPORATIVE COOLING/HEATING SYSTEM WITH ROOF-MOUNTED WATER TANK and U.S. Pat. No. 6,415,620 for DUAL LOOP VEHICLE AIR CONDITIONING SYSTEM incorporated by reference herein and assigned to the assignee of this invention. There are also definite advantages in mounting other equipment and utility components above the roof line, such as, for example batteries to be used for hybrid engines as well as heating and air conditioning systems so long as they are compact enough to meet government height standards. In this same connection, it is desirable to permit interchangeable utilization of roof-mounted battery compartments with other equipment, such as, heater compartments for use in colder climates either with or without roof-mounted radiator compartments. In order to make this possible without mounting of the compartments beneath the roof line or otherwise substantially altering the structure of the vehicle requires extremely low profile compartments so as not to increase the height of the vehicle and particularly larger commercial vehicles above the limits set by the U.S. Department of Transportation. In other respects, it is important that two or more compartments can be so constructed and arranged as to fit together in different combinations to conform to the vehicle structure and to be easily connected into existing accessories on the vehicle. For example, in passenger buses, it is important that the air conditioning system be connected into the duct work which usually extends along opposite sides of the ceiling of the passenger compartment; and to place the radiator section or sections so that their coolant lines can be connected into the engine, transmission and other accessories when desired; and still further to enable mounting of the battery compartment alongside either one of the radiator and air conditioning compartments or heater compartments as well as to facilitate assembly and maintenance of the roof-mounted compartments described.